O'Toole's first major film role was as a jaded beauty pageant contestant in the 1975 satire Smile; she got the role after doing an impression of a "dead cockroach" at the audition.[2] She also appeared as the tutor and girlfriend of Robby Benson's character in the college basketball story One on One in 1977. She co-starred opposite Gary Busey in the 1980 film Foolin' Around. In 1981, she starred in the HBO onstage production of Vanities, as well as in the TV movie Stand By Your Man, which detailed the life of country music legend Tammy Wynette.[2] Later on in 1982, she appeared briefly as Nick Nolte's girlfriend in 48 Hours. That same year, she played Alice Perrin in Cat People, and then in 1983 she played Lana Lang (love interest to Clark Kent/Superman), and single mother of Ricky in Superman III.
In 1985, she co-starred with Barry Manilow in the CBS television movie Copacabana playing Lola La Mar to Manilow's Tony Starr. Also in 1985, she had a starring role as Ms. Edmunds in the original Bridge to Terabithia, and appeared in the TV adaptation of Strong Medicine the following year. In 1987's Cross My Heart, a romantic comedy, O'Toole had a leading role opposite Martin Short.

In October 2001, 18 years after portraying Lana Lang in Superman III, O'Toole returned to the Superman mythos in the role of Martha Kent, Superman's adoptive mother, in the television series Smallville. She remained part of the show's main cast, though at times in the background,[3] until the end of its sixth season. On January 19, 2010, it was announced that O'Toole would be returning to Smallville for at least one episode.[4]

On November 22, 2010, O'Toole played the role of Veronica, a middle-aged woman suffering from a severe case of Alzheimer's, in season 3 episode 7 of the TV series Lie to Me. In March 2013 she appeared in Grey's Anatomy as a school teacher who finds out, after surgery, that she will die from cancer.

O'Toole portrayed Susan Emerson in six episodes of the first two seasons of Halt and Catch Fire.

In 2016, O'Toole returned to the Stephen King realm as boarding house owner Edna Price in 'The Kill Floor' episode of the King mini-series 11.22.63.

In retrospect, O'Toole can date the beginning of her songwriting career to events during a car ride after the September 11 attacks; as her husband Michael McKean describes it, "On September 11, 2001, Annette found herself without an airline to carry her back down to Los Angeles from Vancouver, where she films Smallville. So she drove a rental car down. The two of us drove it back up together, and on the long drive up there, somewhere between Portland and Seattle, she told me she had a tune in her head."[5] The "tune in her head" became "Potato's in the Paddy Wagon", one of three songs the couple wrote for A Mighty Wind,[5] including the Oscar-nominated song "A Kiss at the End of the Rainbow."

O'Toole sang "What Could Be Better?" — a song she and her husband co-wrote — for the 2004 Disney children's album A World of Happiness.[5][6]

On July 7, 2007, O'Toole appeared as a backing singer for her husband's fictional band Spinal Tap at the London leg of the Live Earth concerts. She has also performed on the band's 2009 Unwigged and Unplugged tour, and contributed lyrics to "Short and Sweet" on the 2009 Spinal Tap album Back from the Dead.[citation needed]

In 2011 she starred in CAP21's production of the new musical Southern Comfort, based on the Sundance award-winning documentary, by Dan Collins and Julianne Wick Davis.

Singer-songwriter spouses Annette O'Toole and Michael McKean perform their song "Kiss at the End of the Rainbow" on Employee of the Month in 2016

O'Toole married actor Bill Geisslinger on April 8, 1983;[2] the couple had two daughters, Nell and Anna.[8][9]

O'Toole first met Michael McKean, the man who would later become her second husband, when McKean was working on Laverne & Shirley.[5] O'Toole and McKean became friends in 1997[5] while starring together on Final Justice, a Lifetime movie. They were married in March 1999.[5]

O'Toole and McKean guest-starred together in "Mega", a 10th-season episode of Law & Order as a wealthy married couple accused of murder.[10] The couple also guest-starred on Boy Meets World as Topanga Lawrence's parents. McKean also played reporter Perry White, later the boyfriend of O'Toole's character Martha Kent, on Smallville.

^"Will the Senator From Smallville Please Rise". Zap2it. April 13, 2006. Retrieved 2010-10-01. Annette O'Toole feared Martha Kent would remain in the adventure's background, but that changed when Martha's husband, Jonathan (John Schneider), died shortly after his election to a Kansas senatorial seat. She has stepped up to fill his term, with ever-scheming Lionel Luthor (John Glover) taking a bit too much interest for her comfort.